Prepare for v1.2.0

[akshayjshah]

Update changelog to v1.2.0. The version constant is already at the correct value.

[dgladkov]

Consider making a 2.0.0 release instead.

• Dynamic providers are no longer supported.
• NewValue behavior is completely changed - it used to just build a Value struct, however now it's actually looking up a value from Provider. As custom Provider.Get implementations return a Value constructed from NewValue, this will lead to infinite recursion in most practical use cases.

Because public API was redesigned and old deprecated API is largely backwards incompatible, I feel that 2.0 release is justified. This will also give you opportunity to remove NewProviderGroup and and NewValue.

[akshayjshah]

I hear you. Bugfixes are always changes in behavior, and there were so many bugs in this package that fixing them changed a lot of behavior. The impact of these changes on static YAML providers is minimal, but your dynamic providers will need quite a bit more work to update. (As discussed on your bugfix PR, I encourage you to fork this package if you prefer the current APIs and behavior.)

I'd love to delete the deprecated APIs and release this as a 2.0, but it's more important to me to keep compatibility with the many users of this code pinned to ^1. It's unfortunate that we cut a 1.0 release with so many bugs and warty APIs :/